It is well known that nucleic acids, i.e. deoxy ribonucleic acid (DNA) and ribonucleic acid (RNA) are essential building blocks in living cells. These acids are high molecular weight polymers which are made up of many "nucleotide" units, each such nucleotide unit being composed of a base (a purine or pyrimidine), a sugar (which is either ribose or deoxyribose) and a molecule of phosphoric acid. DNA contains deoxyribose and the bases adenine, guanine, cytosine and thymine (which may be represented as A, G, C and T, respectively). RNA contains ribose instead of deoxyribose and uracil (U) in place of thymine.
The nucleotide units in DNA and RNA are assembled in definite linear sequences which determine specific biological responses. In the normal mammalian cell, DNA replicates itself and also causes the formation of RNA molecules whose nucleotide sequences carry certain information possessed by the DNA. Thus, the RNA may serve as a messenger (mRNA) for the DNA in order to perform certain functions. The RNA also serve to synthesize protein.
According to the well known Watson-Crick model, each nucleic acid molecule consists of two polynucleotide strands coiled about a common axis. The resulting double helix is loosely held together by hydrogen bonds between complementary base pairs in each strand. In the case of DNA, for example, it appears that hydrogen bonds are formed only between adenine (A) and thymine (T) and between guanine (G) and cytosine (C). Hence adrenine (A) and thymine (T) are viewed as complementary bases which bind to each other as A-T. The same is true for guanine (G) and cytosine (C), as G-C.
In a single polynucleotide strand, any sequence of nucleotides is possible. However, once the order of bases in one strand is specified, the exact sequence in the other chain is simultaneously determined due to indicated base pairing. Accordingly, each strand in a DNA or RNA molecule is the complement of the other and each molecule contains two strands. In replication of DNA, the two strands act as a template for the formation of a new chain with complementary nucleotide sequence. The net result is the production of two new strands complementary to each other and each identical with one of the original strands. These may replicate further DNA or RNA, and as programmed in the system involved.
It is known that bacteria and viruses contain nucleic acids, every bacteria and virus species being characterized by its own special nucleic acid sequence. Bacteria contain very large amounts of both DNA and RNA while viruses generally contain only one kind of nucleic acid, either DNA or RNA, but not both. In any case, as noted, the nucleic acid for any particular bacteria or virus species has a characteristic sequence which, stated simply, can be viewed as a "fingerprint" of that particular species.
When a virus invades a normally functioning mammalian cell, the viral nucleic acids (hereinafter called "foreign" nucleic acid for ease of reference) tend to replicate thus, in effect, reproducing more viral nucleic acids and virus and in one way or another destroy or undesirably affect the function of the infected cell and the cell host. Bacterial cells can replicate, grow, and excrete toxic substances in the presence of mammalian cells. The nucleic acids of the bacteria, both DNA and RNA, function independently of the mammalian cellular nucleic acids and may thus be considered "foreign" nucleic acids. There is also the possibility of malfunctioning mammalian cells wherein the cellular nucleic acids or portions of the cellular nucleic acids are replicated or produced in an aberrant manner with consequent undesirable effects on the cell. For example undesired or aberrant cellular DNA and particularly RNA molecules might be produced which could lead to the production of unwanted or malfunctioning proteins. For present purposes, the existance or the expression of nucleic acid resulting from any such malfunctioning is also called "foreign nucleic acid" to differentiate from the DNA and RNA existent or normally expressed in the normally functioning cell.